tests: Update ospf6_ecmp_inter_area topotest (#16197 regression test, unified config)

[gromit1811]

The new test_multipath_inter_area() function reliably checks for #16197 and will fail until that is fixed (the current version of the topotest only fails occasionally). Also, convert topotest to use unified config.

[gromit1811]

In theory, ready for merging. But you might want to wait for the fix for #16197 to avoid permanent CI failures.
Replaces #16797.

[gromit1811]

Does anybody know what this failure here means? https://ci1.netdef.org/browse/FRR-PULLREQ3-CHECKOUT-8369
The missing files really don't exist anymore, I'm using unified config (only frr.conf now)

[riw777]

looks good

[riw777]

CI failure looks related --

 
 	{
 	    "internalNextHopActiveNum": 1,
 	    "nexthops": [
 	        {
 	            "fib": true,
 	            "interfaceName": "r1-eth2",
 	            "active": true
 	        }
 	    ]
 	}```

[gromit1811]

CI failure looks related

That's good. It shows that the new test function is working as expected since the bug it's checking for is still there. The CI failure should disappear once #16197 is fixed (by #18345, which hopefully should be ready for merging in a few days).

[riw777]

CI failure looks related

That's good. It shows that the new test function is working as expected since the bug it's checking for is still there. The CI failure should disappear once #16197 is fixed (by #18345, which hopefully should be ready for merging in a few days).

okay -- let me know when the other one is ready and I'll work on getting these pushed